Did you download something and find a suspicious looking .md file among the contents? Unless you are into finance, it is a markdown file.

A markdown file is just a text file (yes you can open it in any text editor) which has been formatted using the Markdown formatting language. Markdown, like HTML, is a markup language, only it is simpler and easier on the human eyes.

If you open a .md file in a regular text editor, you are likely to see lines starting with #, ##; sentences enclosed within *, **, `, _, __ etc. Those the syntax of the Markdown language. In HTML we have things like <h1>, <strong> etc; in Markdown we have things like #, ** etc. Basically Markdown makes text formatting easier compared to HTML.

In software systems that can parse .md files, the content of markdown files are output in their intended formatting, just like how browsers show the formatted version of the .html files.

Markdown is not the only simpler formatting alternative to HTML, there are many more, like Textile, YAML etc; but I am yet to come across them being distributed like .md files. You can have an edge over you friends and enemies if you know the Markdown syntax, besides it is used by many open source projects for various purposes, so learn it.

So, the next time you see a .md file, fear not and open it with the appropriate program to see its contents.